Many conventional magnetic recording media can be generally referred to as coated type magnetic recording media, which are usually produced by coating the surface of a non-magnetic support with a liquid coating composition prepared by dispersing a magnetic powder of an oxide such as .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped Fe.sub.3 O.sub.4, a berthollide compound of .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, the Co-doped berthollide compound, CrO.sub.2, etc., or a magnetic powder of an alloy of Fe, Ni, Co, etc., in an organic binder such as a vinyl chloridevinyl acetate copolymer, a styrene-butadiene copolymer, an epoxy resin, a polyurethane resin, etc., and drying to form a magnetic layer, as disclosed in U.S. Pat. No. 4,135,016.
Recently, due to an increase in the amount of information to be recorded, there has been a strong desire to make practical use of magnetic recording media suitable for high density recording. Accordingly, there has been keen interest in the so-called thin layer type magnetic recording media prepared by forming a thin ferromagnetic metal layer on the support by vapor deposition, sputtering, ion plating, metal plating, etc., without using a binder, as disclosed in U.S. Pat. Nos. 3,342,632 and 3,342,633. With the progress in the development and research on these magnetic recording media, there have been various proposals regarding their practical use.
However, the foregoing thin layer type magnetic recording media are undesirable because scratches are liable to form on the surfaces due to contact with the magnetic heads, guide rollers, liners, etc. The media is also undesirable because of little mechanical strength, a large coefficient of friction, poor tape running properties and also insufficient weather resistance.
The surface of metal layers is generally very smooth and, thus, the smoothness quantitatively determined by conventional methods would not be practically useful. One approach to determination of smoothness is by evaluating lubricating property and various methods have been proposed, but no effective methods for quantitatively determining the lubricating property are available at present. Such property is generally evaluated totally in terms of tape running properties, but the smoothness of the metal layer does not necessarily correspond to the running property in view of occurrence of stick-slip and the like. On the other hand, the metal layer tends to peel off from the support with many cracks of the metal layer when the tape having a metal layer is passed repeatedly while contacting with magnetic heads, guidepins, etc. Such peeling off can be generally evaluated by test for determining adhesion strength using an adhesive tape.
Various attempts have been made at eliminating these disadvantages by forming overcoats on the magnetic recording media by various systems such as a vacuum vapor deposition system, a coating system, etc. Examples of such methods include forming an overcoat of a solid lubricating agent by sputtering or ionplating as disclosed in U.S. Pat. No. 4,277,540; forming an overcoat of SiO.sub.2 by sputtering as disclosed in U.S. Pat. No. 4,268,369; improving protective properties by liquid treatment as disclosed in U.S. Pat. No. 4,272,563; forming an overcoat by spin-coating of a fluorine-containing compound as disclosed in U.S. Pat. No. 4,268,556; improving the surface of the non-magnetic support by plasma polymerization of perfluorocyclobutane and the like as disclosed in U.S. Pat. No. 4,188,426; and forming an overcoat by plasma poymerization of C.sub.2 F.sub.4 as disclosed in J. Applied Polymer Science, Vol. 23, 2627 (1979). Other examples of methods for forming overcoats include forming an overcoat of a metal such as rhodium, chromium, etc., a high-hardness inorganic material such as WC, TiO.sub.2, CaF.sub.2, MgF.sub.2, etc., or a lubricant such as metal soap, etc., by vapor deposition and a method of forming an overcoat of a polymer by coating. Furthermore, in addition to the foregoing method of forming a single overcoat, there is a method of forming multilayer overcoats each layer of which has a separate function. One such method involves the use of three overcoat layers, the lower overcoat layer being a layer having good adhesion with a substrate or a magnetic recording layer, the intermediate overcoat layer being a layer imparting toughness and high hardness to the magnetic recording medium, and the upper overcoat layer being a layer having excellent smoothness. For the reasons described above, the application of a single overcoat does not give satisfactory results. The application of multilayer overcoats also has disadvantages in that the layer or layers peel apart due to poor adhesion between the layers. Furthermore, the formation of such multilayer overcoats is complicated.